Previous steering systems for land vehicles having a pair of tractor wheels or endless tracks were driven by respective axle shafts powered by a power shaft connected to an engine, a differential gear being interposed between the power shaft and the axle shaft. Each axle shaft is associated with a hydraulic unit that may act both as a hydraulic motor or a hydraulic pump. The two hydraulic units are connected together by a hydraulic circuit that connects the input of a unit to the output of the other unit. A hydraulic pump having an adjustable flow rate is connected to the hydraulic circuit for varying the flow rate of a hydraulic fluid fed to the hydraulic unit, in order to have different speed of rotation of the two hydraulic units and, consequently different speed of rotation of the axle shafts driving the tractor wheels or endless tracks.
These designs however do not incorporate how to pilot the hydraulic pump with the steering device of the vehicle, for instance by a steering wheel.
U.S. Pat. No. 3,371,734 discloses a steering system for a vehicle wherein a pair of endless tracks are driven from a common power shaft at relative speeds which may be selectively varied for turning the vehicle in one direction or the other. The steering system comprises a main power shaft coupled with an engine of the vehicle, a pair of drive shafts driving said endless tracks, transmission means including a pair of differential gear trains for powering said endless tracks, each of said differential gear trains being provided with a first input connected to the main power shaft, a second input and an output connected to the respective drive shaft. The steering system further comprises two control shafts each connected to said second input of a respective differential gear train and a torque generator comprising a hydrostatic pump-and-motor unit connected to said control shafts and responsive to a steering device for applying to said control shafts a reversible differential torque continuously variable in a range between zero and a predetermined maximum.